This proposal describes the development of novel chemical reactions for heterocycle synthesis and the application of these reactions to bioactive natural products. Specifically, a one-pot imine acylation/enol cyclization cascade has been developed that can selectively provide 4-oxazolidinones or pyrrolidones in a stereoselective fashion and these reactions have been applied to the total synthesis of the synoxazolidinones and pretazettine. The synoxazolidinones are recently discovered antimicrobial natural products with novel structures and promising activity against several clinically relevant bacterial strains. Pretazettine is an alkaloid bearing a densely functionalized pyrrolidine core and cytotoxic activit against several types of tumors. The objective of the proposed research is to develop a scalable and expedient approach to these two families of bioactive natural products and leverage the synthetic work to develop new chemical probes and lead compounds for antimicrobial and anticancer development. We have already developed a diastereoselective approach to the synoxazolidinones and prepared analogs with activity superior to that of the natural products. We aim to further refine our synthetic approach and apply it to other members of the synoxazolidinone family while continuing to optimize the 4-oxazolidinone scaffolds for potency and selectivity towards bacteria. In parallel to these efforts we have expanded the scope of our reaction to provide pyrrolidone derivatives, which are broadly useful building blocks in medicinal chemistry and natural product synthesis. Utilizing this approach we propose an extremely rapid preparation of pretazettine, an alkaloid that has received significant attention from the synthetic community. We expect that our synthetic work will provide new reactions to construct heterocycles of broad utility and lead to approaches to natural products that enable their use as chemical probes. These synthetic studies are complimented by chemical biology studies to explore the activity of both natural products and analogs against clinically relevant bacteria and tumor cell lines. Taken together, these efforts will provide a framework for the development of new synthetic chemistry and potential lead compounds for the treatment of bacterial infections and cancer.